I would just like a get eveyone's view on the purposes/"what they do" on a compressor.

They way I understand it is, the threshold is the dB level that is reached by a signal that causes the knee + threshold settings to come in effect and "compress" the signal.

So the knee is a combination of the threshold and ratio. The threshold is the apex of the knee correct? And the ratio is the angle of the knee after that apex? That is a very basic overview but the ratio is the proportion a signal must be compressed once crossing that threshold.

5 Answers
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Threshold is the point set for when the compressor actually starts compressing once the volume of the program or track reaches above that level. Note that it's a static volume, so if you have a voice or instrument that is all over the place due to levels, you might want to do a pre-leveling with a fader/volume automation to even it out so it's always hitting the compressor at the same point on the material - otherwise certain portions will be over-compressed, under-compressed, etc.

The knee is a rounded-off joint at the threshold point that smooths out the transition from non-compression and compression. It can sound very nice on certain sharp and pokey voices. Many compressors call this "soft knee". If you see a graphical representation of it, it rounds off the threshold point so it's round and doesn't have an angle, the more you put it on.

The ratio is how much the compression does - for example, at a ratio of 4:1, for every 4dB of volume past the threshold, the output of the compressor will be 1 dB.

Do you understand how the attack and release portions of the compressor interact with the thresh/knee/ratio?

Great explanation! And of course to extrapolate one step further, anything beyond 10:1 tends to change from being identified as "Compression" to "Limiting" - two sides of the same coin.
– StavrosoundMar 30 '12 at 20:42

attack is how quickly it takes effect and release is how long it decays/or takes the compression to tapper off?
– JM VMar 31 '12 at 9:49

The best explanation of a compressor I've ever read is in Stavro's 'Mixing with your Mind' book.

I don't think it's appropriate to post the actual content here, but the book is definitely worth a read. It's aimed at music, but so many of the techniques he outlines can be transferred to all aspects of audio.